To measure ultrafast phenomena that occur in picosecond (ps) to femtosecond (fs) regions, measurements using an ultrashort pulse laser are carried out widely. For example, in a pump probe reflectance measurement, a high-intensity pump pulse is focused to an object to be measured first to excite the sample instantaneously. Then, while the excited state is being relaxed, an irradiation of a low-intensity probe pulse is applied, and the intensity of the reflected light is measured. The measured intensity of the reflected light is proportional to the reflectance of the sample at the moment when it is hit by the probe pulse. By measuring the intensity of the reflected light while gradually changing the delay time of the probe pulse with respect to the pump pulse, change in the reflectance of the sample between before and after the excitation can be observed with the time resolution comparable to the duration of the laser pulse. This time resolution falls within a picosecond to femtosecond range.
Since the intensity of signals obtained by such ultrahigh-speed measurement is extremely weak in many cases, a modulation measurement is performed to improve the signal-to-noise ratio. As the most common method, the intensity of the pump pulse is modulated, and the response of the probe signal to this modulation is detected by using a lock-in amplifier. In addition, methods in which polarization of the light of the pump pulse or the delay time is modulated are also known.
It is especially noteworthy that the delay time modulation method, in which delay time is modulated as a rectangular wave, has recently been found to be effective in order to realize a time-resolved scanning probe microscope, which is a scanning probe microscope combined with a pulse laser. The time-resolved scanning probe microscope achieving 1-ps temporal resolution and 1-nm spatial resolution at the same time has thus been constructed.
To control the delay time of laser pulse precisely in a time domain from fs to nanosecond (ns), the length of optical path is generally varied.
FIG. 10 is a view illustrating the configuration of a conventional delay time modulation device. As shown in FIG. 10, the conventional delay time modulation device performs a periodic delay time modulation by mechanically vibrating the position of mirror. The laser pulse from the light source is divided by a half mirror 1 (HM1) into two optical paths, namely a path to a retroreflector 1 (RR1) and that to a retroreflector 2 (RR2). It is not necessary that the amount of light at dividing is 1:1. Any ratio can be selected arbitrarily depending on the property of the half mirror to be used. RR1 and RR2 are devices that reflect optical pulses in a direction directly opposite to the incident direction. Retroreflectors usually composed of three mirrors placed so that they make right angles to one another are generally used. The reflected lights from RR1 and RR2 are overlapped precisely on the same optical axis at a half mirror 2 (HM2).
When the length of the optical path passing through RR1 differs from that of the optical path passing through RR2, optical pulses appear at different positions temporally deviated from each other on the optical axis overlapped at HM2. The delay time between the two pulses can be controlled precisely by mechanically varying the position of RR1 or RR2. By using a piezoelectric device, etc., the accuracy of 1 fs or shorter can be achieved. To modulate the delay time periodically using such a device, it is only necessary to periodically change the position of RR1, for example. So far, many measurements have been carried out wherein the mirror position is varied periodically to perform the delay time modulation as described above.
However, the device as shown in FIG. 10 has major limitations in the amplitude of modulation and in the frequency of modulation. The optical path length and the delay time are proportional to each other, with the velocity of light serving as the coefficient. For example, the position of the mirror must be varied with the amplitude of 1.5 cm to modulate the delay time by 100 ps. Such a significant modulation in the mirror position can be achieved only at an extremely low frequency of approximately 10 Hz. In using an amplitude about 1.5 cm or more, or a frequency about 10 Hz, problems may arise. The mechanical vibration may be generated and give bad influences to the optical device located around. It becomes impossible to realize the accurate modulation due to the deformation of the driving mechanism itself.
Meanwhile, since the result of the modulation performed at such a low frequency is strongly affected by the fluctuation in the intensity of laser light, etc., the time-resolved measurement with modulating mirror position is only useful when the modulation amplitude is very small (of up to about 100 fs).
Recently, following the development of a time-resolved scanning probe microscope, which is a scanning probe microscope adopting delay time modulation of a pulse laser, a delay time modulation method using high speed optical shutters (called pulse pickers) capable of passing or blocking optical pulses with respect to each pulse has been proposed, and its usefulness has been confirmed (Patent Literature 1).
FIG. 11 is a time chart illustrating the conventional delay time modulation method using pulse pickers.
A laser oscillator generates laser pulses at time intervals of approximately 10 ns, and these laser pulses are split into two optical paths by a half mirror, etc., and are introduced to respective two pulse pickers from the right-hand side. The pulse picker can selectively transmit one optical pulse at arbitrary timing from the continuous pulse train.
It is therefore possible, as shown in FIG. 11, to generate a delay time by transmitting pulses at different timing. When the delay time is generated by using pulse pickers as described above, the delay time modulation can be performed at extremely high speed and with large amplitude. In the first place, with this method, the minimum value of the amplitude of the modulation delay time is determined by the pulse interval of the optical pulse train, and typically it is approximately 10 ns. This value is equivalent to approximately 3 m in the length of optical path, which is 3 to 4 orders greater than that of the amplitude of the delay time modulation achievable by changing mirror positions. Furthermore, the delay time can be changed for each transmission pulse with this method. Thus, approximately 1 MHz high-speed modulation can be performed as required.
Meanwhile, the delay time modulation using pulse pickers as shown in FIG. 11 cannot produce favorable results when applied to observing fast phenomena of 1 ns or faster. This is because that pulses are picked to generate delay time, thus causing the excitation frequency of the sample, namely the number of times of measurement per unit time, to decrease significantly.
When water-cooled type Pockels cells are used as pulse pickers, for example, it is difficult to increase the repetition frequency of the output optical pulses to higher than 2 MHz due to the restriction imposed by the generation of heat from the Pockels cells. Whereas the repetition frequency of the conventional titanium-sapphire laser oscillator is generally close to 100 MHz, the number of times of excitation of the sample per unit time becomes 1/50, and also the number of detected signals becomes 1/50, when pulse pickers are used.